The invention relates to a process, and to apparatus, for applying a liquid coating to at least one surface of an article. The process produces a high quality, attractive and uniform coating on both flat and contoured surfaces, while minimizing waste of the liquid coating. Preferred articles to be coated using the process are paper plates, cartons, bowls, serving dishes, trays, or similar articles made of molded pulp, pressed or folded paperboard, and the like. While these articles are preferred, essentially any type of article may be coated using the disclosed process and apparatus.
Some manufactured articles can be improved by application of a coating on at least one surface of the article. For example, molded pulp articles used for serving food, such as molded pulp plates and bowls, are light-weight, disposable and low cost. In use, however, these articles can lose their form and/or function in the presence of heat, moisture and/or grease (for example, as in the case of a paper plate weakening when hot, wet food is held in the plate). A protective barrier on a molded pulp or paper article can prevent loss of strength or rigidity due to such factors, which improves the performance of the article in immediate use, and in applications where food is held with or without refrigeration in the article for a period of time, and possibly heated, prior to consumption. Other reasons for coating an article include providing additional surface strength, improving cut resistance, or adding to the aesthetic appearance of the article. For example, one may add color, patterns, textures and sheen to the surface of the article. One may also coat an article to protect an underlying print or to improve its printability.
It is known to pre-apply a wax or polymer coat on the food-contact surface of paperboard, and to thereafter form the paperboard into useful food-service articles. There are problems, however, when attempting to apply coatings to preformed, contoured molded pulp articles rather than to flat paperboard.
GB 1,111,525 (published May 1, 1968) describes some disadvantages of prior art efforts to coat articles, particularly articles with deep contours. For example, dipping an entire article in liquid coating can saturate the article and apply too much coating to the article. Furthermore, it is impractical to dip only the inside (food-contacting) surface of a deeply contoured article into a liquid coating material.
It is also known to spray paper and molded pulp articles with liquid coating materials. See, for example, Canadian Patent Application 2,057,474. Aqueous dispersions or latexes of various plastics, such as acrylic, vinyl and styrene-butadiene, have been sold for many years for use in coating molded pulp articles. Spraying a contoured article with liquid coating, however, can produce uneven coatings, despite the exercise of care in the spray application. Moreover, spray-coating methods used in the prior art generally do not make efficient use of the coating material. Overall, the prior art of coating articles with a protective barrier layer leaves room for improvement.